The 20-day limit on holding children has been in place since 2015, which rose out of a 1997 court-ordered consent decree after a federal class action lawsuit alleged the conditions of detention facilities are unsuitable for children.
The American Civil Liberties Union (ACLU) condemned the policy, saying: "The government should not be jailing kids, and certainly shouldn't be seeking to put more kids in jail for longer". Border apprehensions have slumped more than 40% since May, a major drop officials have attributed to increased enforcement in Mexico, but officials worry about a resurgence in the fall.
Homeland security officials believe that time limits on the detentions of migrant families have driven the surge of Central Americans crossing the border into the United States this year.
Presidential candidate Beto O'Rourke, a former congressman from a district along the border in Texas, noted in a Facebook post Wednesday that seven migrant children have died in the previous year either in US custody or soon after release.
McAleenan said under the new rule, "All children in the governments care will be universally treated with dignity, respect, and special concern in concert with American values - and faithful to the intent of the original settlement".
New regulations to be announced would be "a pathway to ensure the humane detention of families while satisfying the goals" of the Flores Settlement, DHS said.
Immigrant advocates decried the move and said prolonged detention would traumatise immigrant children.
The latest measure is the administration's most aggressive and legally questionable effort to curtail legal protections for undocumented children since its much-criticized "zero tolerance" policy resulted in the separation of thousands of migrant families.
The core principle and requirement of the Flores Agreement is that migrant children taken into detention should be released as "expeditiously" as possible.
Jill Biden: ‘Swallow a little bit’ and vote for Joe Former Vice-President Biden has dropped his first television ad in Iowa, and it touches on his strongest selling point. Trump said on Sunday that "there's something going on at Fox" and that his "worst polls have always been from Fox".
The new rule announced by acting Homeland Security Secretary Kevin McAleenan would allow the children of illegal-alien families to be held beyond the current 20-day limit.
They argue that the new regulation will counter the belief that bringing children into the U.S. is "a passport" to being freed from detention after a short period.
DHS officials said expanding family detention helped reduce family crossings under the Obama administration. Most have been released in the United States, officials said.
The government operates three family detention centers that can hold a total of about 3,000 people, though one is being used for single adults, and the other two are at capacity.
At a detention center in McAllen, Texas, there were almost 1,800 juveniles when the entire capacity for both juveniles and adults was 1,500.
McAleenan said the government believes some families apprehended on the border were "fraudulent" based on DNA testing of some migrants in pilot programmes implemented in recent months. The number of individuals within family units who Border Patrol has apprehended along the southern border so far this fiscal year reached more than 390,000 by the end of June.
McAleenan said he didn't expect to need more bed space because, together with other efforts to restrict the flow of migrants, he expects fewer people coming.
President Donald Trump and his officials targeted the Flores agreement after their widely condemned "zero tolerance" policy previous year failed to deter border crossings. Parents went to criminal court and then immigration detention, while their children were sent to federal shelters. That agreement is generally interpreted as meaning families must be released within 20 days.
The rule was slated to take effect in 60 days. Children cared for toddlers, the lawyers said, adding that they had inadequate food, water and sanitation.